Air heaters with tubular nests resisting the corrosion caused by the condensation of the fume acid vapours are known (see notably FR-A-2,454,598-2,545,197), in which the incoming air is subjected in the first tubes of the nest to a preheating at a high temperature, in such manner that the temperature of the tubes is maintained at a value which is higher than that of the acid dew point of the fumes. However, these known solutions do not prevent corrosion caused by the sodium and vanadium salts since preheating the air flowing in the first tubes of the tubular nest subjected to the action of the corrosive fumes maintains these first tubes at a temperature which is above the corrosion critical temperature of sodium/vanadium.
One will understand that in order to avoid, or at least reduce, simultaneously the corrosion hazards due to the sulfur oxidized components and the sodium and vanadium salts, it is imperative to maintain the skin of the heater tubes in contact with the fumes, on the one hand at a temperature which is below the corrosion critical temperature of sodium/vanadium, and on the other hand at a temperature which is above the acid dew point of the fumes.
In an attempt to solve this problem, air heaters with tubular nests are provided including a nest made of corrosion resisting tubes and fed with air at a temperature and at a flow rate such that the temperature of the skin of these tubes remains below the corrosion critical temperature of the sodium and vanadium salts. On the other hand, in order to reduce the corrosion hazards due to the acid condensations caused by the sulfur oxides, the final passes, in the direction of the flow of the fumes, can be effected in a cocurrent way (meaning that the two fluids: fumes and air to be heated, flow in the same direction), thereby avoiding a too low temperature on the tube skins.
According to this solution, one will understand that the design of the air heaters is fixed from the very start and that it is not possible to thereafter adapt the design to the various operating conditions usually encountered in the equipment mounted upstream. This fixed design is therefore a major disadvantage since it not only causes possible throughout drops but it does not allow total supression of corrosion hazards in the extreme cases of maximum and minimum operation.